Is there such a thing as a second hand motor home guide such as Glasses for cars, or a web site where I can find out how much my vehicle is worth before I put it on the market?
I've never come across anything like that in the UK, Julia. One of the French MH magazines pubishes one, annually, but their market is MUCH bigger - as is our car market.
Motorhomes are still a bit of a "niche" here, so the chances of finding enough similar vans for sale in the same year to arrive at an "average" price are pretty remote - even for popular models.
So it's really a matter of checking out other vans on the market as similar to yours as possible, bearing in mind that what people are ASKING isn't necessarily what they'll GET.
Ultimately, of course, your van is "worth" what someone is willing to pay for it, and what you're willing to accept. But it's easier to predict the former when the market is larger!
When we needed to agree the insurance value when our first van was written off we agreed with the company that we would obtain examples of similar vans on the market. We (both sides) were realistic enough to acknowledge that forecourt prices had some wriggle room but also to not place too much emphasis on the lower price bracket (because of higher mileage &c). In the end we agreed on something around the middle of 5 or 6 vans on sale at the time which both sides were satisfied with.
Those were dealer selling prices of course. Private sale prices would be a bit less as with cars.
Motorhomes vary so much, each one is different, as owners place their stamp on them. also different usage , wear and tear, as you are living in Mh, where as a car you are just sitting in and driving from a to b. mileage IMO is not an issue, as high mileage is not a bad thing on a diesel engine. When looking for a secondhand Mh , look for layout, that suite your style of living, as mistakes can be costly